Generally speaking, stereotactic radiosurgery is concerned with radiation therapy for small intracranial volumes and, for example, for arteriovenous malformations, or for tumors. It makes use chiefly, at the present time, of two different techniques that have proved their worth for many years, a dynamic technique and a static technique.
The dynamic technique involves a single source capable of producing a narrow beam of ionizing radiation that is mobile in space in relation to the target volume to be treated.
This first technique makes use of devices mainly constructed on the basis of linear accelerators, and ionizing radiation is mostly produced by a source of high-energy photons.
The static technique makes involves a plurality of ionizing beams which, during treatment, are static in relation to the target volume to be treated, being sharply collimated and focused on one and the same irradiation isocenter. The invention falls within the field of the aforementioned second, static technique, which will now be described in greater detail.
A stereotactic radiosurgery device using the static technique has already been described, for example, in French patent application FR 2 672 220 and in U.S. Pat. No. 4,780,898. Such a device comprises a plurality of sources of ionizing radiation and, for example, radioactive sources of gamma radiation, of the .sup.60 Co sources, which are mounted on a hemispherical device facing a plurality of primary collimators, there being one source for each primary collimator. A helmet internal to the aforementioned hemispherical device is fitted with smaller diameter secondary, removable collimators and enables a plurality of isocentric mini-beams to be obtained.
Prior to implementing the treatment, a stereotactic frame is placed on the patient's skull, this frame serving to locate the volume to be treated, known as the `target volume`, in the mechanical coordinate system of the treatment device, using an appropriate medical imaging modality (essentially, X-ray angiography for arteriovenous malformations and Computed Tomography or Magnetic Resonance Imaging in the case of tumoral lesions). The same stereotactic frame is used to position the patient's skull in relation to the helmet of the stereotactic radiosurgery device, in such a way that the irradiation isocenter of the helmet is in a known position in relation to the target volume.
To effect a shot at a given point on the target volume, known as the `target point`, the patient's skull is positioned in such a way that the irradiation center of the helmet coincides with the target point. When all the collimators on the helmet are of the same diameter, the spatial distribution of the dose obtained in a volume assumed to be homogenous is substantially spherical and centered on the isocenter, the maximum dose being delivered at the isocenter, and the dose delivered at a distance from the isocenter equal to the radius of the collimators substantially amounting to 50% of the maximum dose received by the isocenter. In the static radiosurgery technique, the treatment of a target volume is thus comparable with a punching out operation, during which it is attempted to juxtapose spatially the doses delivered at each shot in such a way as to cover the target volume in its entirety.
In usual practice, as a function of a predetermined target volume for treatment, an operator specialized in radiosurgery decides on a treatment plan, by defining, in a first stage, the number of shots to be effected, and the target point of each shot, that is to say the point on the target volume on which, for a given shot, the irradiation center has to be positioned, and, in a second stage, the configuration of the helmet for each shot. The configuration of the helmet is to be taken here as referring to the diameter of the secondary collimators that have to be mounted on the helmet, and the treatment time for a given shot, that is to say the duration of the shot. The operator can thus decide that a single-target plan, that is to say a single shot on a single target point, is sufficient, if he considers that a single shot will enable a sufficient dose to be delivered throughout the target volume, or, on the contrary, decide on a multi-target plan, effecting a series of successive shots on predetermined target points. The treatment plan and the configuration, or successive configurations, of the helmet must be chosen not only in order to cover the entire target volume with the optimum treatment dose, but also, when the target volume is positioned in the vicinity of a sensitive area, for example in the vicinity of the optic chiasma, taking care to ensure that the dose delivered in this sensitive area, and, generally speaking, outside the target volume, be as small as possible. For this purpose, the operator takes as his basis a certain number of predetermined points for which he knows the optimum dose that ought to be delivered at each of these points these will be, for example, points on the envelope of the target volume, certain points on the inside of the target volume, and, if applicable, certain sensitive points placed outside and in the vicinity of the target volume, and for which the dose must be as small as possible.
Hitherto, and in practice, a specialized operator has had several helmets with which to define his treatment plan, each helmet comprising a number of collimators equal to the number of collimators that can be fitted on the helmet. The collimators for each helmet are of identical or different diameters from one helmet to another. The configuration of the helmet for each shot (diameter of the collimators on the helmet and shot duration) is determined empirically. In order, as a preliminary measure, to validate his choice of configurations and, if applicable, to change it, the operator is provided with a software that enables him, on the basis of each helmet configuration, to effect automatically a three-dimensional calculation of the dose resulting from the set of shots in the case of a multi-target plan, or from a single shot in the case of a single-target plan. The relevance of the configurations for a given treatment plan is gauged by comparing the three-dimensional distribution of the calculated dose with the target volume and, as applicable, with the sensitive volumes.
There are several drawbacks in empirically choosing helmet configurations. The choice necessarily has to be made by an operator specialized in radiosurgery, on the basis of his experience. It leads, in practice, to a three-dimensional dose geometry which is not best suited to the target volume, which target volume can have any non-spherical contour; as a result, to cover the target volume in its entirety while, at the same time, avoiding, as far as possible, irradiating the area of healthy tissue in the external vicinity of the target volume, the operator has, in practice, to carry out, almost systematically, a series of several shots, knowing that he has a limited choice of collimator diameters. Now, each shot necessitates considerable time for treatment and for positioning the patient, which leads to substantial operating overheads and discomfort to the patient. From a financial viewpoint, and to ensure the patient's comfort, it thus proves necessary to limit the number of shots, and even to be able to reduce the treatment plan to a single shot.